On the Origin of Phyla By James Valentine, University of Chicago Press, $55, ISBN 0226845486
THE most important event in the history of life is still a mystery. In rocks of the Cambrian age, laid down in the sea more than 520 million years ago, animal fossils suddenly become abundant, and within a few million years representatives of many of the major groups of animals that still dominate the world had appeared. Yet go back another 30 million years to rocks from the late Precambrian and there is little fossil evidence of the kind of animals that might be plausible ancestors. There are few intermediates.
Living animals, the descendants of the Cambrian explosion, are classified by zoologists into more than 30 phyla 鈥� nobody can agree exactly how many 鈥� each corresponding to a different body design. They are amazingly diverse yet share many features of morphology and genes, so there is no question that they descended step-by-step from a common ancestor.
Advertisement
The problem that James Valentine addresses is how to infer these hidden and mysterious evolutionary steps. How are animals related one to another? And why do they appear as fossils so suddenly, an event known as the Cambrian explosion?
Valentine brings together a mass of evidence from many sources in a magisterial compendium, On the Origin of Phyla. There is direct evidence from the fossil record, of course. But there is also new evidence from trees of relationships derived from studying similarities in gene sequences of living animals. These are often ambiguous. Then there is evidence from the way genes are expressed during the growth and development of the body plans of animals 鈥� the growing field of 鈥渆vo-devo鈥� that seeks to explain the balance between evolution and development. Add to this classical morphology and embryology 鈥� which was once seen as the key to unlock life鈥檚 secrets but turns out to be as difficult to interpret as any other evidence. It is an astonishing range of information all brought together within one pair of covers. It鈥檚 enough to make mere mortals awestruck.
Valentine鈥檚 title deliberately invites comparison with Darwin鈥檚 On the Origin of Species. Does it measure up as a comparably seminal work? Frankly, no. There is no new, single driving idea that explains the tempo and mode of the mysterious explosion. What Valentine offers is a judicious evaluation of an astonishing array of evidence. The science is still a work in progress. But this book will be an essential tool for anyone who takes a serious interest in one of the most intractable episodes in our planet鈥檚 long history.